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Abstract:

An offset transistor and a non-offset transistor each including an oxide
semiconductor are formed over one substrate. An oxide semiconductor
layer, a gate insulator, and first layer wirings which serve as gate
wirings are formed. After that, the offset transistor is covered with a
resist and impurities are mixed into the oxide semiconductor layer, so
that an n-type oxide semiconductor region is formed. Then, second layer
wirings are formed. Through the above steps, the offset transistor and
the non-offset transistor (e.g., aligned transistor) can be formed.

Claims:

1. A semiconductor device comprising a first transistor and a second
transistor, wherein the first transistor comprises: a first gate
electrode; a first gate insulating layer adjacent to the first gate
electrode; a first oxide semiconductor layer adjacent to the first gate
electrode with the first gate insulating layer interposed therebetween;
and a first source electrode and a first drain electrode in contact with
the first oxide semiconductor layer, wherein the second transistor
comprises: a second gate electrode; a second gate insulating layer
adjacent to the second gate electrode; a second semiconductor layer
adjacent to the second gate electrode with the second gate insulating
layer interposed therebetween; and a second source electrode and a second
electrode in contact with the second oxide semiconductor layer, wherein
one of the first source electrode and the first drain electrode is
connected to one of the second source electrode and the second drain
electrode, and wherein, when seen from above, the first source electrode
and the first drain electrode are separated from the first gate
electrode.

2. The semiconductor device according to claim 1, wherein, when seen from
above, the second source electrode and the second drain electrode are
separated from the second gate electrode.

3. The semiconductor device according to claim 1, further comprising a
third transistor comprising a third gate electrode, wherein the third
gate electrode is connected to the other of the second source electrode
and the second drain electrode.

4. The semiconductor device according to claim 1, wherein the first gate
electrode is connected to the second gate electrode.

5. The semiconductor device according to claim 1, wherein each of the
first oxide semiconductor layer and the second oxide semiconductor layer
includes indium and zinc.

6. A semiconductor device comprising a first transistor, a second
transistor, a first capacitor, and a second capacitor, wherein the first
transistor comprises: a first gate electrode; a first gate insulating
layer adjacent to the first gate electrode; a first oxide semiconductor
layer adjacent to the first gate electrode with the first gate insulating
layer interposed therebetween; and a first source electrode and a first
drain electrode in contact with the first oxide semiconductor layer,
wherein the second transistor comprises: a second gate electrode; a
second gate insulating layer adjacent to the second gate electrode; a
second semiconductor layer adjacent to the second gate electrode with the
second gate insulating layer interposed therebetween; and a second source
electrode and a second electrode in contact with the second oxide
semiconductor layer, wherein one of the first source electrode and the
first drain electrode is connected to one of the second source electrode
and the second drain electrode, wherein one of the second source
electrode and the second drain electrode is connected to the first
capacitor, wherein the other of the second source electrode and the
second drain electrode is connected to the second capacitor, and wherein,
when seen from above, the first source electrode and the first drain
electrode are separated from the first gate electrode.

7. The semiconductor device according to claim 6, wherein, when seen from
above, the second source electrode and the second drain electrode are
separated from the second gate electrode by a space.

8. The semiconductor device according to claim 6, further comprising a
third transistor comprising a third gate electrode, a third source
electrode, and a third drain electrode, wherein the third gate electrode
is connected to the other of the second source electrode and the second
drain electrode.

9. The semiconductor device according to claim 6, wherein the first gate
electrode is connected to the second gate electrode.

10. The semiconductor device according to claim 6, wherein the oxide
semiconductor includes indium and zinc.

11. The semiconductor device according to claim 6, further comprising a
lip-flop circuit, wherein the other of the second source electrode and
the second drain electrode is connected to the flip-flop circuit.

Description:

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a semiconductor including an oxide
semiconductor. Note that here, semiconductor devices refer to general
elements and devices which function utilizing semiconductor
characteristics. For example, a memory, an arithmetic circuit, a
rectifier, a display device, and the like which include a semiconductor
can be given as semiconductor devices; however, the present invention is
not limited thereto. For example, Patent Document 1 can be referred to
for a memory including a semiconductor.

[0003] 2. Description of the Related Art

[0004] In recent years, a transistor, a display device, a memory, and the
like manufactured using a composite oxide of indium, gallium, and zinc
have been reported (for example, see Patent Documents 2 to 5).

[0010] An object of one embodiment of the present invention is to provide
a novel integrated circuit including at least two transistors which are
formed using an oxide semiconductor and a method for manufacturing the
integrated circuit. Other objects will be apparent from and can be
derived from the description of the specification, the drawings, the
claims, and the like.

[0011] Structures which can solve the above problems are described below.
Before the description of the structures, terms used in this
specification are described. Note that in this specification and the
like, a transistor is an element including at least three terminals,
i.e., a gate, a drain, and a source. In addition, the transistor has a
channel region between a drain (a drain terminal, a drain region, or a
drain electrode) and a source (a source terminal, a source region, or a
source electrode), and current can flow through the drain, the channel
region, and the source.

[0012] Here, since the source and the drain of the transistor change
depending on the structure, the operating condition, and the like of the
transistor, it is difficult to define which is a source or a drain. Thus,
a portion which functions as the source and a portion which functions as
the drain are not called a source and a drain and one of the source and
the drain is referred to as a first electrode and the other thereof is
referred to as a second electrode in some cases.

[0013] Also in the case of an element having two terminals, such as a
capacitor or a diode, one electrode is referred to a first electrode and
the other electrode is referred to as a second electrode in some cases.
In this case, even when a positive electrode and a negative electrode are
distinguished from each other in the capacitor or the diode, "the first
electrode" does not indicate whether the one electrode is the positive
electrode or is the negative electrode. However, when it is necessary to
specify the positive electrode and the negative electrode because of the
characteristics of the circuit, description is additionally made in some
cases.

[0014] Note that in this specification and the like, terms such as
"first", "second", and "third" are used for distinguishing various
elements, members, regions, layers, and areas from others. Therefore, the
terms such as "first", "second", "third", and the like do not limit the
number of the elements, members, regions, layers, areas, or the like.
Further, for example, it is possible to replace "first" with "second",
"third", or the like.

[0015] Note that in this specification and the like, when it is explicitly
described that X and Y are connected, the case where X and Y are
electrically connected, the case where X and Y are functionally
connected, and the case where X and Y are directly connected are included
therein. Here, each of X and Y denotes an object (e.g., a device, an
element, a circuit, a wiring, an electrode, a terminal, a conductive
film, a layer, or the like). Accordingly, another connection relation
shown in drawings and texts is included without being limited to a
predetermined connection relation, for example, the connection relation
shown in the drawings and the texts.

[0016] For example, in the case where X and Y are electrically connected,
one or more elements which enable electrical connection between X and Y
(e.g., a switch, a transistor, a capacitor, an inductor, a resistor,
and/or a diode) can be connected between X and Y.

[0017] Note that when it is explicitly described that X and Y are
electrically connected, the case where X and Y are electrically connected
(i.e., the case where X and Y are connected with another element or
another circuit provided therebetween), the case where X and Y are
functionally connected (i.e., the case where X and Y are functionally
connected with another circuit provided therebetween), and the case where
X and Y are directly connected (i.e., the case where X and Y are
connected without another element or another circuit provided
therebetween) are included therein. That is, when it is explicitly
described that "X and Y are electrically connected", the description is
the same as the case where it is explicitly only described that "X and Y
are connected".

[0018] Note that in this specification and the like, it might be possible
for those skilled in the art to constitute one embodiment of the
invention even when portions to which all the terminals of an active
element (e.g., a transistor), a passive element (e.g., a capacitor), or
the like are connected are not specified. In particular, in the case
where the number of portions to which the terminal is connected might be
plural, it is not necessary to specify the portions to which the terminal
is connected. Thus, it might be possible to constitute one embodiment of
the invention by specifying only portions to which some of terminals of
an active element, a passive element, or the like are connected.

[0019] Note that in this specification and the like, it might be possible
for those skilled in the art to specify the invention when at least the
connection portion of a circuit is specified. Alternatively, it might be
possible for those skilled in the art to specify the invention when at
least a function of a circuit is specified.

[0020] Therefore, when a connection portion of a circuit is specified, the
circuit is disclosed as one embodiment of the invention even when a
function is not specified, and one embodiment of the invention can be
constituted. Alternatively, when a function of a circuit is specified,
the circuit is disclosed as one embodiment of the invention even when a
connection portion is not specified, and one embodiment of the invention
can be constituted.

[0021] Note that in this specification and the like, explicit singular
forms preferably mean singular forms. However, without being limited
thereto, such singular forms can include plural forms. Similarly,
explicit plural forms preferably mean plural forms. However, without
being limited thereto, such plural forms can include singular forms.

[0022] One embodiment of the present invention is a semiconductor device
which includes a first transistor and a second transistor each including
an oxide semiconductor. A second electrode of the first transistor and a
first electrode of the second transistor are connected to each other. The
first transistor has a gate, a first electrode, and the second electrode
which are in an offset state (hereinafter, referred to as an offset
transistor).

[0023] One embodiment of the present invention is a semiconductor device
which includes a first transistor including an oxide semiconductor, a
first capacitor, a second transistor including an oxide semiconductor,
and a second capacitor. A second electrode of the first transistor, a
first electrode of the second transistor, and a first electrode of the
first capacitor are connected to one another. A second electrode of the
second transistor is connected to a first electrode of the second
capacitor. A second electrode of the first capacitor and a second
electrode of the second capacitor are connected to nodes different from a
gate of the first transistor and a gate of the second transistor. The
first transistor is an offset transistor.

[0024] One embodiment of the present invention is a semiconductor device
which includes a first transistor including an oxide semiconductor, a
capacitor, a second transistor including an oxide semiconductor, and a
flip-flop circuit. A second electrode of the first transistor, a first
electrode of the capacitor, and a gate of the second transistor are
connected to one another. A second electrode of the capacitor is
connected to a node different from a gate of the first transistor. A
first electrode of the second transistor is connected to the flip-flop
circuit.

[0025] In the above semiconductor device, the second transistor may be a
transistor whose gate overlaps with a first electrode or a second
electrode (hereinafter, referred to as an overlap transistor). In the
above semiconductor device, the second transistor may be an offset
transistor.

[0026] One embodiment of the present invention is a method for
manufacturing a semiconductor device that includes the following steps:
forming a first electrode of a first transistor and a second electrode of
a second transistor; forming an oxide semiconductor film; forming a gate
of the first transistor, a gate of the second transistor, a second
electrode of a first capacitor, and a second electrode of a second
capacitor; forming a mask covering at least the first electrode and the
gate of the first transistor; and mixing an impurity into the oxide
semiconductor film. The first electrode and the gate of the first
transistor, and the second electrode and the gate of the second
transistor are in an offset state.

[0027] One embodiment of the present invention is a method for
manufacturing a semiconductor device that includes the following steps:
forming an oxide semiconductor film; forming a gate of a first transistor
and a gate of a second transistor; forming a mask covering at least the
gate of the first transistor; mixing an impurity into the oxide
semiconductor film; forming a first electrode of the first transistor and
a second electrode of the second transistor; and forming a second
electrode of a first capacitor and a second electrode of a second
capacitor. The first electrode and the gate of the first transistor are
in an offset, and the second electrode and the gate of the second
transistor are in an offset.

[0028] An oxide semiconductor to be used preferably contains at least
indium (In) or zinc (Zn). In particular, In and Zn are preferably
contained. As a stabilizer for reducing variation in electric
characteristics of a transistor including the oxide semiconductor,
gallium (Ga) is preferably additionally contained. Tin (Sn) is preferably
contained as a stabilizer. Hafnium (Hf) is preferably contained as a
stabilizer. Aluminum (Al) is preferably contained as a stabilizer.

[0030] As the oxide semiconductor, for example, any of the following can
be used: indium oxide; tin oxide; zinc oxide; a two-component metal oxide
such as an In--Zn-based oxide, a Sn--Zn-based oxide, an Al--Zn-based
oxide, a Zn--Mg-based oxide, a Sn--Mg-based oxide, an In--Mg-based oxide,
or an In--Ga-based oxide; a three-component metal oxide such as an
In--Ga--Zn-based oxide (also referred to as IGZO), an In--Al--Zn-based
oxide, an In--Sn--Zn-based oxide, a Sn--Ga--Zn-based oxide, an
Al--Ga--Zn-based oxide, a Sn--Al--Zn-based oxide, an In--Hf--Zn-based
oxide, an In--La--Zn-based oxide, an In--Ce--Zn-based oxide, an
In--Pr--Zn-based oxide, an In--Nd--Zn-based oxide, an In--Sm--Zn-based
oxide, an In--Eu--Zn-based oxide, an In--Gd--Zn-based oxide, an
In--Tb--Zn-based oxide, an In--Dy--Zn-based oxide, an In--Ho--Zn-based
oxide, an In--Er--Zn-based oxide, an In--Tm--Zn-based oxide, an
In--Yb--Zn-based oxide, or an In--Lu--Zn-based oxide; a four-component
metal oxide such as an In--Sn--Ga--Zn-based oxide, an
In--Hf--Ga--Zn-based oxide, an In--Al--Ga--Zn-based oxide, an
In--Sn--Al--Zn-based oxide, an In--Sn--Hf--Zn-based oxide, or an
In--Hf--Al--Zn-based oxide.

[0031] Here, for example, an In--Ga--Zn-based oxide means an oxide
containing In, Ga, and Zn as its main component, and there is no
particular limitation on the ratio of In, Ga, and Zn. Further, the
In--Ga--Zn-based oxide may contain a metal element other than In, Ga, and
Zn.

[0032] Alternatively, a material represented by InMO3(ZnO), (m>0
and m is not an integer) may be used as the oxide semiconductor. Here, M
represents one or more metal elements selected from Ga, Fe, Mn, and Co.
Alternatively, as the oxide semiconductor, a material represented by
In2SnO5(ZnO), (n>0 and n is an integer) may be used.

[0033] For example, an In--Ga--Zn-based oxide with an atomic ratio of
In:Ga:Zn=1:1:1 (=1/3:1/3:1/3) or 2:2:1 (=2/5:2/5:1/5) or any of oxides
whose composition is in the neighborhood of the above compositions can be
used. Alternatively, an In--Sn--Zn-based oxide with an atomic ratio of
In:Sn:Zn=1:1:1 (=1/3:1/3:1/3), 2:1:3 (=1/3:1/6:1/2), or
2:1:5(=1/4:1/8:5/8), or any of oxides whose composition is in the
neighborhood of the above compositions may be used.

[0034] However, the present invention is not limited to the above
compositions, and a material having an appropriate composition may be
used depending on necessary semiconductor characteristics (e.g.,
mobility, threshold voltage, and variation). In order to obtain necessary
semiconductor characteristics, it is preferable that the carrier
concentration, the impurity concentration, the defect density, the atomic
ratio of a metal element to oxygen, the interatomic distance, the
density, and the like be set to appropriate values.

[0035] For example, with an In--Sn--Zn-based oxide, a high mobility can be
obtained with relative ease. However, mobility can be increased by
reducing the defect density in a bulk also in the case of using the
In--Ga--Zn-based oxide.

[0036] Note that for example, the expression "the composition of an oxide
containing In, Ga, and Zn at the atomic ratio, In:Ga:Zn=a:b:c (a+b+c=1),
is in the neighborhood of the composition of an oxide containing In, Ga,
and Zn at the atomic ratio, In:Ga:Zn=A:B:C (A+B+C=1) means that a, b, and
c satisfy the following relation:
(a-A)2+(b-B)2+(c-C)2≦r2, and r may be 0.05,
for example. The same applies to other oxides.

[0037] The oxide semiconductor may be either single crystal or
non-single-crystal. In the latter case, the oxide semiconductor may be
either amorphous or polycrystal. Further, the oxide semiconductor may
have either an amorphous structure including a crystalline portion or a
non-amorphous structure.

[0038] In the case of an oxide semiconductor in an amorphous state, a flat
surface can be obtained with relative ease, so that when a transistor is
manufactured with the use of such an oxide semiconductor, interface
scattering can be reduced, and relatively high mobility can be obtained
with relative ease.

[0039] In an oxide semiconductor having crystallinity, defects in the bulk
can be further reduced and when surface flatness is improved, mobility
higher than that of an oxide semiconductor in an amorphous state can be
obtained. In order to improve the surface flatness, the oxide
semiconductor is preferably formed over a flat surface.

[0040] Specifically, the oxide semiconductor may be formed over a surface
with an average surface roughness (Ra) of less than or equal to 1
nm, preferably less than or equal to 0.3 nm, further preferably less than
or equal to 0.1 nm.

[0041] Note that Ra is obtained by expanding, into three dimensions,
center line average roughness that is defined by JIS B 0601 so as to be
applied to a plane. The Ra can be expressed as an "average value of
the absolute values of deviations from a reference surface to a
designated surface" and is defined by the following formula.

[0042] In the above formula, S0 represents the area of a plane to be
measured (a quadrangular region which is defined by four points
represented by coordinates (x1, y1), (x1, y2),
(x2, y1), and (x2, y2)), and Z0 represents an
average height of the plane to be measured. Ra can be measured using
an atomic force microscope (AFM).

[0043] In the case where the oxide semiconductor film has crystallinity,
an oxide semiconductor film including a crystal with c-axis alignment
(also referred to as a c-axis aligned crystalline oxide semiconductor
(CAAC-OS) film), which has a triangular or hexagonal atomic arrangement
when seen from the direction of an a-b plane, a surface, or an interface
may be used. In the crystal, metal atoms are arranged in a layered
manner, or metal atoms and oxygen atoms are arranged in a layered manner
along the c-axis, and the direction of the a-axis or the b-axis is varied
in the a-b plane (the crystal rotates around the c-axis).

[0044] In a broad sense, a CAAC-OS means a non-single-crystal oxide
semiconductor including a phase which has a triangular, hexagonal,
regular triangular, or regular hexagonal atomic arrangement when seen
from the direction perpendicular to the a-b plane and in which metal
atoms are arranged in a layered manner or metal atoms and oxygen atoms
are arranged in a layered manner when seen from the direction
perpendicular to the c-axis direction.

[0045] The CAAC-OS is not a single crystal but this does not mean that the
CAAC-OS is composed of only an amorphous component. Although the CAAC-OS
includes a crystallized portion (a crystalline portion), a boundary
between one crystalline portion and another crystalline portion is not
clear in some cases.

[0046] In the case where the CAAC-OS includes oxygen, nitrogen may be
substituted for part of oxygen included in the CAAC-OS. The c-axes of
crystalline portions included in the CAAC-OS may be aligned in a certain
direction (e.g., a direction perpendicular to a surface of a substrate
over which the CAAC-OS is formed or a surface of the CAAC-OS).
Alternatively, the normals of the a-b planes of the crystalline portions
included in the CAAC-OS may be aligned in a certain direction (e.g., a
direction perpendicular to a surface of a substrate over which the
CAAC-OS is formed or a surface of the CAAC-OS).

[0047] The CAAC-OS can be a conductor, a semiconductor, or an insulator
depending on its composition or the like. Further, the CAAC-OS transmits
or does not transmit visible light depending on its composition or the
like.

[0048] An example of such a CAAC-OS film is a crystal which has a
triangular or hexagonal atomic arrangement when observed from the
direction perpendicular to a surface of the film or a surface of a
substrate over which the CAAC-OS is formed, and in which metal atoms are
arranged in a layered manner or metal atoms and oxygen atoms (or nitrogen
atoms) are arranged in a layered manner when a cross section of the film
is observed.

[0049] An offset transistor has lower off-state current (leakage current
flowing between a source and a drain in an off state) than a non-offset
transistor. Further, the non-offset transistor has higher on-state
current (current flowing between a source and a drain in an on state)
than the offset transistor.

[0050] Those are described based on results of numerical calculation using
gate voltage (potential difference between a source and a gate, VG)
dependence of a drain current (current between a source and a drain,
ID) of a transistor whose channel includes an ideal oxide
semiconductor without a defect inside the semiconductor.

[0051] As the oxide semiconductor, an oxide semiconductor in which the
ratio of indium (In), tin (Sn), and zinc (Zn) is 1:1:1 is assumed. Before
the numerical calculation, a transistor is manufactured using an oxide
having such a composition, and it is found that the defect density in the
bulk is approximately 1×1012/cm2 and that the mobility in
the case where no defect exists in the bulk is 120 cm2Ns owing to
its characteristics.

[0052] Note that even when no defect exists inside a semiconductor,
scattering at an interface between a channel and a gate insulator affects
the transport property of the transistor. In other words, the mobility
μ1 at a position that is distance x away from the interface
between the channel and the gate insulator can be expressed as the
following formula.

1 μ 1 = 1 μ 0 + D B exp ( - x G )
[ Formula 2 ] ##EQU00002##

[0053] Here, D represents the electric field in the gate direction, and B
and G are constants. B and G can be obtained from actual measurement
results; according to the above measurement results, B is
4.75×107 cm/s and G is 10 nm (the depth to which interface
scattering reaches). When D is increased (i.e., when the gate voltage is
increased), the second term of Formula 2 is increased and accordingly the
field-effect mobility μ1 is decreased.

[0054] Based on the above results, FIG. 7 shows calculation results of
drain current only in consideration of interface scattering. Note that a
potential of a source is 0 V, and a potential of a drain is +1 V. For the
numerical calculation, device numerical calculation software Sentaurus
Device manufactured by Synopsys, Inc. is used, and the relative
permittivity, the bandgap, the electron affinity, and the thickness of
the oxide semiconductor are assumed to be 15, 2.8 eV, 4.7 eV, and 15 nm,
respectively. These values are obtained by measurement of a thin film
that is formed by sputtering.

[0055] FIGS. 6A and 6B illustrate cross-sectional structures of the
transistors used for the numerical calculation. The transistors
illustrated in FIGS. 6A and 6B each include a semiconductor region 603a
and a semiconductor region 603c that have n-type conductivity in an oxide
semiconductor layer. The resistivities of the semiconductor regions 603a
and 603c are 2×10-3 Ωcm.

[0056] The transistor illustrated in FIG. 6A is formed over a base
insulator 601 and an embedded insulator 602 which is embedded in the base
insulator 601 and formed of aluminum oxide. The transistor includes the
semiconductor region 603a, the semiconductor region 603c, an intrinsic
semiconductor region 603b serving as a channel formation region
therebetween, and a gate 605. The width of the gate 605 is 33 nm A gate
insulator 604 is formed between the gate 605 and the semiconductor region
603b. In addition, a sidewall insulator 606a and a sidewall insulator
606b are formed on both side surfaces of the gate 605, and an insulator
607 is formed over the gate 605 to prevent a short circuit between the
gate 605 and another wiring. The width of the sidewall insulator is 5 nm.
A source 608a and a drain 608b are provided in contact with the
semiconductor region 603a and the semiconductor region 603c,
respectively. Note that the channel width of this transistor is 40 nm.

[0057] In the numerical calculation, the work functions of the gate 605,
the source 608a, and the drain 608b are assumed to be 5.5 eV, 4.6 eV, and
4.6 eV, respectively. The thickness of the gate insulator 604 is assumed
to be 100 nm, and the relative dielectric constant thereof is assumed to
be 4.1. The channel length and the channel width are 33 nm and 40 nm,
respectively.

[0058] The transistor in FIG. 6B is the same as the transistor in FIG. 6A
in that it is formed over the base insulator 601 and the embedded
insulator 602 formed of aluminum oxide and that it includes the
semiconductor region 603a, the semiconductor region 603c, the intrinsic
semiconductor region 603b provided therebetween, the gate 605 having a
width of 33 nm, the gate insulator 604, the sidewall insulator 606a, the
sidewall insulator 606b, the insulator 607, the source 608a, and the
drain 608b.

[0059] The transistor in FIG. 6A is different from the transistor in FIG.
6B in the conductivity type of semiconductor regions under the sidewall
insulator 606a and the sidewall insulator 606b. In the transistor in FIG.
6A, the semiconductor regions under the sidewall insulator 606a and the
sidewall insulator 606b are part of the semiconductor region 603a having
n-type conductivity and part of the semiconductor region 603c having
n-type conductivity, whereas in the transistor in FIG. 6B, the
semiconductor regions under the sidewall insulator 606a and the sidewall
insulator 606b are part of the intrinsic semiconductor region 603b.

[0060] In other words, a region (offset region) having a width of
Loff which overlaps with neither the semiconductor region 603a
(semiconductor region 603c) nor the gate 605 is provided. The width
Loff is called an offset length. As is seen from the drawing, the
offset length is equal to the width of the sidewall insulator 606a
(sidewall insulator 606b). Note that the transistor in FIG. 6A has a
width Loff of 0 nm and is neither an offset transistor nor an
overlap transistor. In this specification, such a transistor is called an
aligned transistor.

[0061] Actually, it is impossible to obtain a structure with neither
overlap nor offset, and slight overlap or offset is inevitable; however,
an island transistor may used as a transistor having a structure between
overlap and offset.

[0062] In FIG. 7, a dotted line indicates characteristics of a transistor
having the structure in FIG. 6A (aligned transistor, Loff=0 nm), and
a solid line indicates characteristics of a transistor having the
structure in FIG. 6B (offset transistor, Loff=15 nm).

[0063] Since an oxide semiconductor has a bandgap of greater than or equal
to 2.5 eV, the number of thermally excited carriers is small and
extremely high resistance can be obtained in an off state. However,
unlike in a silicon semiconductor, channel doping to suppress a
short-channel effect cannot be performed; therefore, the drain current is
greater than or equal to 1 pA at a gate voltage of 0 V.

[0064] The offset transistor (FIG. 6B) has lower off-state current than
the aligned transistor (FIG. 6A). As compared to the transistor in FIG.
6A, the drain current is smaller by 3 orders of magnitude at a gate
voltage of 0 V and smaller by 6 or more orders of magnitude at a gate
voltage of -1 V. However, on-state current of an offset transistor is
lower than that of a non-offset transistor. At a gate voltage of +1 V,
the drain current of the transistor in FIG. 6B is approximately 1/3 of
that of the transistor in FIG. 6A.

[0065] With a structure according to one embodiment of the present
invention, an offset transistor and a non-offset transistor (e.g., an
aligned transistor or an overlap transistor) can be formed over one
substrate at the same time. In other words, in a circuit, a non-offset
transistor can be used in a portion where high on-state current is
preferable, and an offset transistor can be used in a portion where low
off-state current is preferable.

BRIEF DESCRIPTION OF THE DRAWINGS

[0066] In the accompanying drawings:

[0067] FIGS. 1A to 1D illustrate examples of cross-sectional schematic
views and a circuit of semiconductor devices of embodiments of the
present invention;

[0068] FIGS. 2A to 2D are cross-sectional views illustrating manufacturing
steps of a semiconductor device of one embodiment of the present
invention;

[0069] FIGS. 3A and 3B each illustrate a circuit of a semiconductor device
of one embodiment of the present invention;

[0070] FIGS. 4A to 4C are cross-sectional views illustrating manufacturing
steps of a semiconductor device of one embodiment of the present
invention;

[0071] FIGS. 5A and 5B are cross-sectional views illustrating
manufacturing steps of a semiconductor device of one embodiment of the
present invention;

[0072] FIGS. 6A and 6B each illustrate a cross-sectional structure of a
transistor used for numerical calculation;

[0073]FIG. 7 shows numerical calculation results of characteristics of a
transistor including an oxide semiconductor; and

[0074] FIGS. 8A to 8C each illustrate an example of an electronic device.

DETAILED DESCRIPTION OF THE INVENTION

[0075] Hereinafter, embodiments will be described with reference to
drawings. However, the embodiments can be implemented with various modes.
It will be readily appreciated by those skilled in the art that modes and
details can be changed in various ways without departing from the spirit
and scope of the present invention. Thus, the present invention should
not be interpreted as being limited to the following description of the
embodiments.

[0076] Size, the thickness of layers, or regions in drawings are
exaggerated for simplicity in some cases. Therefore, the embodiments of
the present invention are not limited to such scales.

[0077] Note that drawings are schematic views of ideal examples, and the
embodiments of the present invention are not limited to the shape or the
value illustrated in the drawings. For example, variation in shape due to
a manufacturing technique or dimensional deviation can be included.

[0078] Further, technical terms are often used in order to describe a
specific embodiment, example, or the like. Note that one embodiment of
the invention is not construed as being limited by the technical terms.

[0079] In addition, terms which are not defined (including terms used for
science and technology, such as technical terms or academic parlance) in
this specification can be used as terms which have meaning equal to
general meaning that an ordinary person skilled in the art understands.
It is preferable that terms defined by dictionaries or the like be
construed as consistent meaning with the background of related art.

[0080] Note that what is described (or part thereof) in one embodiment can
be applied to, combined with, or exchanged with another content in the
same embodiment and/or what is described (or part thereof) in another
embodiment or other embodiments.

Embodiment 1

[0081]FIG. 1A is a cross-sectional view of a semiconductor device of this
embodiment. The semiconductor device includes a first oxide semiconductor
layer 102a and a second oxide semiconductor layer 102b over a substrate
101. Further, a first layer wiring 103a is provided in contact with the
first oxide semiconductor layer 102a, a first layer wiring 103b is
provided in contact with the second oxide semiconductor layer 102b, and a
first layer wiring 103c is provided in contact with the first oxide
semiconductor layer 102a and the second oxide semiconductor layer 102b.

[0082] A gate insulator 104 is provided over the first oxide semiconductor
layer 102a, the second oxide semiconductor layer 102b, and the first
layer wirings 103a to 103c. A second layer wiring 105a, a second layer
wiring 105b, a second layer wiring 105c, and a second layer wiring 105d
are provided over the gate insulator 104.

[0083] The semiconductor device includes two transistors and two
capacitors. The second layer wiring 105b serves as a gate of a first
transistor, and the first layer wiring 103b serves as a first electrode
of the first transistor. The second layer wiring 105a serves as a gate of
a second transistor, and the first layer wiring 103a serves as a second
electrode of the second transistor. The first layer wiring 103c serves as
a second electrode of the first transistor and a first electrode of the
second transistor. The first transistor is an offset transistor whose
gate does not overlap with a first electrode and a second electrode, and
the second transistor is an overlap transistor whose gate overlaps with a
first electrode and a second electrode.

[0084] The second layer wiring 105c serves as a second electrode of a
first capacitor. The second layer wiring 105d serves as a second
electrode of a second capacitor. The first layer wiring 103a also serves
as a first electrode of the second capacitor. The first layer wiring 103c
also serves as a first electrode of the first capacitor.

[0085]FIG. 1D illustrates a circuit including the two transistors and the
two capacitors. Here, Tr1 represents the first transistor; Tr2,
the second transistor; Cs1, the first capacitor; and Cs2, the
second capacitor. The gate of the first transistor Tr1 and the gate
of the second transistor Tr2 are connected to one signal terminal
CLK, and the first transistor Tr1 and the second transistor Tr2
operate in conjunction with each other. For example, the first electrode
of the first transistor Tr1 is connected to an input terminal IN,
and an input signal input to the first electrode of the first transistor
Tr1 is held in a storage node SN that is a node of the first
electrode of the second capacitor Cs2. Note that the gate of the
first transistor Tr1 and the gate of the second transistor Tr2
may be apart from each other and supplied with different signals.

[0086] The first capacitor Cs1 and the second capacitor Cs2 may
be capacitance which is unintentionally formed, such as capacitance
between wirings or parasitic capacitance, instead of capacitance which is
intentionally formed.

[0087]FIG. 1B illustrates a variation of a semiconductor device. A
semiconductor device in FIG. 1B is the same as the semiconductor device
in FIG. 1A except that the second transistor is an offset transistor.

[0088]FIG. 1c illustrates a variation of a semiconductor device. In a
semiconductor device in FIG. 1c, one first oxide semiconductor layer 102
is provided, whereas two oxide semiconductor layers (the first oxide
semiconductor layer 102a and the second oxide semiconductor layer 102b)
are provided in the semiconductor device in FIG. 1A. The other structures
are the same as those in the semiconductor device in FIG. 1A.

Embodiment 2

[0089] In this embodiment, a method for manufacturing an offset transistor
and an aligned transistor over one oxide semiconductor layer will be
described with reference to FIGS. 2A to 2D.

[0090] An oxide semiconductor layer 202 is formed over a substrate 201. A
variety of substrates can be used as the substrate 201. For example, a
single crystal silicon wafer, a glass substrate, a silicon on insulator
(SOI) substrate, or the like can be used. The substrate 201 preferably
has an insulating surface, and in the case where a semiconductor
substrate or a conductive substrate is used, an insulating layer
(hereinafter, referred to as a first insulating layer) is preferably
provided on its surface. A circuit may be formed over the substrate 201,
and the first insulating layer may be provided over the circuit.

[0091] The first insulating layer is preferably formed using an oxide, and
it is much preferable that the first insulating layer include excessive
oxygen. The hydrogen concentration of the first insulating layer is
preferably sufficiently low and is more preferably lower than or equal to
1×1019 cm-3. For that purpose, it is preferable that at
least a surface portion of the first insulating layer be formed by a
sputtering method in an atmosphere in which the hydrogen concentration is
sufficiently reduced. Note that silicon oxide is preferably used because
the first insulating layer preferably has a low dielectric constant.

[0092] It is preferable that the surface of the first insulating layer be
sufficiently flat, and the average surface roughness be less than or
equal to 1 nm, preferably less than or equal to 0.3 nm, more preferably
less than or equal to 0.1 nm. In particular, in the case where the oxide
semiconductor layer 202 which is formed over the first insulating layer
has crystallinity and the surface of the first insulating layer is not
sufficiently flat, the crystallinity of the oxide semiconductor layer 202
is insufficient in some cases. Insufficient flatness causes variation in
transistor characteristics. In order to obtain a flat surface, the
surface of the first insulating layer is preferably planarized by a
chemical mechanical polishing method. It is still preferable to perform
plasma treatment on the surface of the first insulating layer after the
planarization by a chemical mechanical polishing method.

[0093] The oxide semiconductor layer 202 is preferably formed using any of
the above oxide semiconductors, and the thickness thereof is determined
in consideration of the size of a transistor. The thickness of the oxide
semiconductor layer 202 may be greater than or equal to 1 nm and less
than or equal to 30 nm, for example. Alternatively, when a channel length
is denoted by L, the thickness of the oxide semiconductor layer 202 may
be greater than or equal to 1% and less than 10% of L.

[0094] The oxide semiconductor layer 202 can be obtained in such a manner
that an oxide semiconductor film is etched to a necessary shape. The
oxide semiconductor film is preferably formed by a sputtering method in
an atmosphere including an oxygen gas at a substrate heating temperature
of higher than or equal to 100° C. and lower than or equal to
600° C., preferably higher than or equal to 150° C. and
lower than or equal to 550° C., and more preferably higher than or
equal to 200° C. and lower than or equal to 500° C. In the
case where a mixed atmosphere of an oxygen gas and a rare gas is used,
the percentage of the oxygen gas is 30 vol. % or more, preferably 50 vol.
% or more, and more preferably 80 vol. % or more.

[0095] As the substrate heating temperature at the time of film formation
is higher, the impurity (hydrogen and the like) concentration of the
obtained oxide semiconductor film is lower. Further, the atomic
arrangement in the oxide semiconductor film is ordered and the density
thereof is increased, so that a crystal is easily formed. The donor
concentration of the oxide semiconductor film is preferably lower than or
equal to 1×1011 cm-3.

[0096] Next, the gate insulator 203 is formed. The thickness of the gate
insulator 203 is determined in consideration of the size of the
transistor. The thickness of the gate insulator 203 may be greater than
or equal to 5 nm and less than or equal to 30 nm, for example.
Alternatively, when the channel length is denoted by L, the thickness of
the gate insulator 203 may be less than 10% of L. The thickness of the
gate insulator 203 is determined also in consideration of the dielectric
constant, and when a material having a high dielectric constant is used,
the thickness of the gate insulator 203 can be increased.

[0097] Note that it is further preferable that a dielectric constant
.di-elect cons.1 and a thickness t1 of the oxide semiconductor
layer 202, a dielectric constant .di-elect cons.2 and a thickness
t2 of the gate insulator 203, and the channel length L of the
transistor satisfy (.di-elect cons.2t1+.di-elect
cons.1t2)<0.1.di-elect cons.2L.

[0098] The gate insulator 203 can be formed by a sputtering method, an
evaporation method, a PCVD method, a PLD method, an ALD method, or an MBE
method. Silicon oxide, silicon nitride, silicon oxynitride, aluminum
oxide, tantalum oxide, hafnium oxide, zirconium oxide, yttrium oxide, or
the like may be used for the gate insulator 203. In this embodiment, a
silicon oxide film is formed to a thickness of 100 nm by a sputtering
method.

[0099] Next, a first conductive film for forming first layer wirings
serving as gate electrodes is formed. The first conductive film can be
formed using a metal material such as molybdenum, titanium, tantalum,
tungsten, aluminum, copper, chromium, neodymium, or scandium, or an alloy
material containing any of these materials as its main component.

[0100] The first conductive film may have a stacked-layer structure. For
the lowermost layer, a metal oxide containing nitrogen, specifically, an
In--Ga--Zn-based oxide containing nitrogen, an In--Sn-based oxide
containing nitrogen, an In--Ga-based oxide containing nitrogen, an
In--Zn-based oxide containing nitrogen, tin oxide containing nitrogen,
indium oxide containing nitrogen, or a metal nitride film (InN, SnN, or
the like), may be used.

[0101] These materials each have a work function of 5 eV or higher,
preferably 5.5 eV or higher. Thus, any of these materials used for the
gate electrode makes the threshold voltage of the transistor positive, so
that a so-called normally-off switching element can be provided.

[0102] Further, a second insulating layer is formed over the first
conductive film. For example, silicon oxide, silicon nitride, silicon
oxynitride, aluminum oxide, or the like is formed. A material of the
second insulating layer is preferably a material which serves as an
etching stopper in a later anisotropic etching step.

[0103] Next, the first conductive film and the second insulating layer are
processed through a photolithography process, whereby a first layer
wiring 204a, a first layer wiring 204b, and an etching stopper 205a and
an etching stopper 205b which are respectively provided over the first
layer wiring 204a and the first layer wiring 204b are formed. A resist
206 is formed in a region where the offset transistor is to be provided
(on the right side in FIG. 2A).

[0104] The oxide semiconductor layer 202 is doped with impurities using
the resist 206, the first layer wiring 204a, and the etching stopper 205a
provided over the first layer wiring 204a as masks. As examples of
impurities to be used, phosphorus, boron, and nitrogen can be given;
however, the impurities are not limited thereto. A substance which
combines with oxygen contained in the oxide semiconductor and increases
oxygen vacancies in the oxide semiconductor and the donor concentration
of the oxide semiconductor may be used.

[0105] In this manner, n-type oxide semiconductor regions 207 are formed.
The donor concentration of the n-type oxide semiconductor region 207 is
preferably higher than or equal to 1×1020 cm-3. At this
time, the boundaries of the n-type oxide semiconductor regions 207 are
roughly aligned with both ends of the first layer wiring 204a.

[0106] After that, a third insulating layer is formed by a plasma CVD
method or the like and subjected to anisotropic etching, whereby sidewall
insulators 208 are formed. A known semiconductor technique (sidewall
formation technique) can be referred to for this step. At this time, the
gate insulator 203 is etched to expose the n-type oxide semiconductor
regions 207 except portions under the sidewall insulators 208 and the
first layer wirings 204a and 204b.

[0107] A second conductive film is formed. A material of the second
conductive film may be selected from materials which can be used for the
first conductive film. The second conductive film is etched, so that a
second layer wiring 209a, a second layer wiring 209b, and a second layer
wiring 209c are obtained. The second layer wiring 209a and the second
layer wiring 209b are in contact with the n-type oxide semiconductor
regions 207.

[0108] Through this step, a main part of the semiconductor device of this
embodiment is formed. In other words, the first layer wiring 204b serves
as the gate of the first transistor (offset transistor), the second layer
wiring 209c serves as the first electrode of the first transistor, and
the second layer wiring 209b serves as the second electrode of the first
transistor. The first layer wiring 204a serves as the gate of the second
transistor (aligned transistor), the second layer wiring 209b serves as
the first electrode of the second transistor, and the second layer wiring
209a serves as the second electrode of the second transistor.

[0109] After that, capacitor insulators 210 are formed. The capacitor
insulators 210 are used as dielectrics of capacitors. For that purpose,
the thickness and the dielectric constant of the capacitor insulators are
set as appropriate. For example, the thickness of the gate insulator 203
and the material of the gate insulator 203 may be referred to for the
thickness and the dielectric constant of the capacitor insulators 210. In
addition, a contact hole or the like may be provided in part of the
capacitor insulator 210 as necessary.

[0110] After that, a third layer wiring 211a, a third layer wiring 211b,
and a third layer wiring 211c are formed. The third layer wiring 211a and
the second layer wiring 209a form a capacitor, and the third layer wiring
211b and the second layer wiring 209b form a capacitor. The third layer
wiring 211a serves as a second electrode of a second capacitor, and the
third layer wiring 211b serves as a second electrode of a first
capacitor. Note that the second layer wiring 209a serves as a first
electrode of the second capacitor, and the second layer wiring 209b
serves as a first electrode of the first capacitor. The third layer
wiring 211c is in contact with the second layer wiring 209c.

[0111] Further, a protective film or an interlayer insulator may be
provided. In this manner, the offset transistor and the aligned
transistor can be formed over the oxide semiconductor layer 202. Note
that an overlap transistor or an offset transistor may be formed instead
of the aligned transistor.

Embodiment 3

[0112] In this embodiment, an example where the circuit in FIG. 1D is used
will be described. FIG. 3A illustrates a circuit of a memory element
including the circuit in FIG. 1D. This memory element includes a circuit
including a first transistor 301, a second transistor 302, a first
capacitor 304, and a second capacitor 305 (a portion surrounded by a
dotted line in the drawing) and a read transistor 303. The portion
surrounded by the dotted line in the drawing is similar to the circuit in
FIG. 1D.

[0113] The first transistor 301 is an offset transistor, and the second
transistor 302 is any of an aligned transistor, an overlap transistor,
and an offset transistor. Each of the first transistor 301 and the second
transistor 302 includes an oxide semiconductor layer as described in
Embodiment 1 or 2.

[0114] When data is input, a potential of a signal terminal CLK of gates
of the first transistor 301 and the second transistor 302 is controlled
so that the first transistor 301 and the second transistor 302 are turned
on. Then, an input terminal IN has a potential based on data. As a
result, potentials of the first electrodes of the first capacitor 304 and
the second capacitor 305 (a potential of the storage node SN) can be set
to potentials based on data.

[0115] The state of the read transistor 303 changes depending on the
potential of the storage node SN of this circuit. In other words, when
the potential of the storage node SN is high, the read transistor 303 is
turned on in some cases, and when the potential of the storage node SN is
low, the read transistor 303 is turned off. Therefore, held data can be
determined by examination of conducting states of a first terminal T1 and
a second terminal T2.

[0116] A memory element having a similar structure is known as a gain cell
(for example, see Patent Document 1). The memory element of the circuit
in FIG. 3A can hold data for a long time by including the first
transistor 301 that is an offset transistor with low off-state current,
as compared to a normal gain cell.

[0117]FIG. 3B illustrates a circuit of another memory element including
the circuit in FIG. 1D. This memory element includes a circuit including
a first transistor 317, a second transistor 318, a first capacitor 320,
and a second capacitor 321 (a portion surrounded by a dotted line in the
drawing) and a read transistor 319. The portion surrounded by the dotted
line in the drawing is similar to the circuit in FIG. 1D. In addition,
the portion surrounded by the dotted line and the read transistor 319 are
similar to the circuit in FIG. 3A.

[0118] The first transistor 317 is an offset transistor, and the second
transistor 318 is any of an aligned transistor, an overlap transistor,
and an offset transistor. Each of the first transistor 317 and the second
transistor 318 includes an oxide semiconductor layer as described in
Embodiment 1 or 2.

[0119] The circuit in FIG. 3B includes a first inverter including a
p-channel transistor 311 and an n-channel transistor 313, a second
inverter including a p-channel transistor 312 and an n-channel transistor
314, and a first switching transistor 315 and a second switching
transistor 316 which are connected to the first inverter and the second
inverter. These form a flip-flop circuit. Such a circuit is used for a
register or the like in a CPU, for example.

[0120] When data is written, the input terminal IN has a potential based
on the data, a third terminal T3 has an appropriate high potential, a
fourth terminal T4 has an appropriate low potential, and a potential of a
signal terminal CLK1 of a gate of the first switching transistor 315 and
a potential of a signal terminal CLK2 of a gate of the second switching
transistor 316 are controlled, whereby the first switching transistor 315
and the second switching transistor 316 are turned on. The data is kept
in a circuit, which has feedback loop, formed by the first inverter and
the second inverter. After that, the first switching transistor 315 is
turned off.

[0121] The stored data can be determined by reading a potential of an
output terminal OUT. It needs to be noted that the potential of the
output terminal OUT has a phase opposite to that of the potential
supplied to the input terminal IN. For example, in the case where input
data is "1" (the potential of the input terminal IN is +2 V) when the
potential supplied to the input terminal IN at data of "1" is +2 V and
the potential supplied to the input terminal IN at data of "0" is 0 V,
the potential of the output terminal OUT is 0 V.

[0122] The above steady state is held in the case where the potentials of
the third terminal T3 and the fourth terminal T4 are kept; however, when
a potential difference between the third terminal T3 and the fourth
terminal T4 is decreased, data is lost. In other words, in the case of
using only the flip-flop circuit, a considerable amount of power is
always consumed for holding data. However, by saving data in the portion
surrounded by the dotted line in the drawing, power consumption of the
memory element can be reduced.

[0123] In order to save data, the potential of a signal terminal CLK3 of
the gates of the first transistor 317 and the second transistor 318 is
controlled in a state where the flip-flop circuit is kept in the steady
state so that the first transistor 317 and the second transistor 318 are
turned on. As a result, the potential of the storage node SN is set to a
potential based on the held data. In the above example, when the held
data is "1", the potential of the storage node SN is 0 V, and when the
held data is "0", the potential of the storage node SN is +2 V.

[0124] After that, the potential of the signal terminal CLK3 of the gates
of the first transistor 317 and the second transistor 318 is controlled
so that the first transistor 317 and the second transistor 318 are turned
off. It is preferable that the first switching transistor 315 and the
second switching transistor 316 be also turned off.

[0125] The potential of the storage node SN is held for a long time with
the use of the first transistor 317 that is an offset transistor with low
off-state current. Note that the potential of the storage node SN is
changed from the initial value in some cases as time passes.

[0126] While data is saved in the above circuit, the potential of the
signal terminal CLK3 may be lower than potentials of the other portions
of the circuit. Thus, the potential of the storage node SN can be held
for a longer time. There is an extremely small amount of current consumed
in that case, and most current is gate leakage of the first transistor
317 and the second transistor 318 (leakage current between the gate and a
drain and between the gate and a source), which is difficult to measure.

[0127] The saved data is restored as follows. First, the second switching
transistor 316, the first transistor 317, and the second transistor 318
remain off. The first switching transistor 315 is turned on, and the
potential of the input terminal IN is set to a potential based on data of
"1" (+2 V in the above example). A potential of a fifth terminal T5 is
preferably set to +2 V as well. Each of the third terminal T3 and the
fourth terminal T4 is set to a predetermined potential. Thus, a potential
of a gate of the first inverter is +2 V. After that, the first switching
transistor 315 is turned off.

[0128] Next, the potential of the fifth terminal is set to 0 V. The read
transistor 319 is off when the potential of the storage node SN is 0 V
(or a potential close to 0 V); therefore, the potential of the gate of
the first inverter remains +2 V. However, the read transistor 319 is on
when the potential of the storage node SN is +2 V (or a potential close
to +2 V); therefore, the potential of the gate of the first inverter is
set to 0 V.

[0129] When the held data is "1", the potential of the storage node SN is
0 V (or a potential close to 0 V), and when the held data is "0", the
potential of the storage node SN is +2 V (or a potential close to +2 V).
Therefore, when the held data is "1", the potential of the gate of the
first inverter is +2 V, and when the held data is "0", the potential of
the gate of the first inverter is 0 V. This state is the same as the
state in the case where the data is first input.

[0130] When the second switching transistor 316 is turned on in that
state, the flip-flop circuit is in a steady state on the basis of the
potential of the gate of the first inverter. This state is the same as
the state before the data is saved.

[0131] Such a memory element may be formed in the following manner: the
first inverter including the p-channel transistor 311 and the n-channel
transistor 313, the second inverter including the p-channel transistor
312 and the n-channel transistor 314, and the first switching transistor
315 and the second switching transistor 316 which are connected to the
first inverter and the second inverter are formed over a silicon wafer by
a known semiconductor manufacture technique; and the first transistor 317
and the second transistor 318 are formed using an oxide semiconductor,
over the circuit.

[0132] The read transistor 319 may be formed in the same layer as the
transistor included in the flip-flop circuit. When the read transistor
319 is formed in the same layer as the first transistor 317 and the
second transistor 318, a circuit can be formed in the same area as a
conventional memory element; thus, the integration degree is not
decreased.

[0133] Note that since the flip-flop circuit requires an area of 50
F2 or more when the minimum feature size is F, the channel length of
each of the first transistor 317 and the second transistor 318 may be 5
times or more as long as the channel length of the transistor included in
the flip-flop circuit. In the case where the first transistor 317 and the
second transistor 318 each have a long channel, the off-state
characteristics can be improved.

Embodiment 4

[0134] In this embodiment, a method that can be used for manufacturing the
memory element in FIG. 3B will be described with reference to FIGS. 4A to
4C and FIGS. 5A and 5B. Note that cross-sectional views illustrated in
FIGS. 4A to 4C and FIGS. 5A and 5B do not illustrate specific cross
sections of the memory element.

[0135] First, an n-well 401n, a p-well 401p, an element separation
insulator 402, an n-type region 403n, a p-type region 403p, a first layer
wiring 404a, and a first layer wiring 404b are formed over a substrate
401 (e.g., a silicon wafer) by a known technique for manufacturing a
semiconductor integrated circuit (see FIG. 4A). The first layer wiring
404a and the first layer wiring 404b serve as gates of transistors.

[0136] Further, a first interlayer insulator 405 is formed, a contact hole
is formed, and a first contact plug 406a, a first contact plug 406b, and
a first contact plug 406c are formed (see FIG. 4B).

[0137] Furthermore, a second interlayer insulator 407, a second layer
wiring 408a, a second layer wiring 408b, a second layer wiring 408c, and
a second layer wiring 408d are formed. The steps up to here can be
performed by a known technique for manufacturing a semiconductor
integrated circuit. Note that the second interlayer insulator 407 and the
second layer wirings 408a to 408d each preferably have a sufficient flat
surface as described in Embodiment 2. In addition, the second interlayer
insulator 407 is preferably formed using a material similar to that of
the first insulating layer in Embodiment 2.

[0138] After that, an oxide semiconductor layer 409, a gate insulator 410,
a third layer wiring 411a, a third layer wiring 411b, a third layer
wiring 411c, and a third layer wiring 411d are formed (see FIG. 4c).
Embodiment 2 may be referred to for formation of these.

[0139] Here, the third layer wiring 411b and the third layer wiring 411d
serve as gate wirings of the transistors. In this embodiment, the third
layer wiring 411b and the third layer wiring 411d are provided so as not
to overlap with the second layer wirings 408a to 408d in portions where
the third layer wirings 411b and 411d overlap with the oxide
semiconductor layer 409.

[0140] In other words, as illustrated in the drawing, the second layer
wiring 408b and the third layer wiring 411b, the second layer wiring 408c
and the third layer wiring 411b, the second layer wiring 408c and the
third layer wiring 411d, and the second layer wiring 408d and the third
layer wiring 411d are set in an offset state with an appropriate offset
length.

[0141] A portion where an offset transistor is to be manufactured is
covered with a resist 412 and impurities are mixed into the oxide
semiconductor layer 409, so that an n-type oxide semiconductor region 413
is formed (see FIG. 5A). Ends of the n-type oxide semiconductor regions
413 are roughly aligned with ends of the third layer wiring 411b;
therefore, an aligned transistor can be formed. Embodiment 2 can be
referred to for the details of this step.

[0142] Further, a third interlayer insulator 414 is formed (see FIG. 5B).
As described above, a first transistor 415d which is an offset
transistor, a second transistor 415b which is an aligned transistor, a
first capacitor 415c, and a second capacitor 415a can be formed.

[0143] The first transistor 415d, the second transistor 415b, the first
capacitor 415c, and the second capacitor 415a can be used as the first
transistor 301, the second transistor 302, the first capacitor 304, and
the second capacitor 305 which are illustrated in FIG. 3A or the first
transistor 317, the second transistor 318, the first capacitor 320, and
the second capacitor 321 which are illustrated in FIG. 3B.

[0144] A transistor including the n-type region 403n, the p-type region
403p, the first layer wiring 404a, the first layer wiring 404b, and the
like can be used as the p-channel transistor 311, the p-channel
transistor 312, the n-channel transistor 313, the n-channel transistor
314, the first switching transistor 315, or the second switching
transistor 316 in FIG. 3B. The transistor is formed in multiple layers as
described above, whereby excellent characteristics can be obtained
without reduction in integration degree of the circuit.

Embodiment 5

[0145] In this embodiment, the case where any of the semiconductor devices
described in the above embodiments is applied to an electronic device
will be described with reference to FIGS. 8A to 8C. In this embodiment,
the cases where any of the above-described semiconductor devices is
applied to electronic devices such as a computer, electronic paper, and a
television device (also referred to as a TV or a television receiver)
will be described.

[0146]FIG. 8A illustrates a notebook personal computer 700, which
includes a housing 701, a housing 702, a display portion 703, a keyboard
704, and the like. At least one of the housings 701 and 702 is provided
with the semiconductor device including the memory element described in
any of the above embodiments. Thus, a notebook personal computer with
sufficiently low power consumption, in which data can be stored for a
long time, can be obtained.

[0147]FIG. 8B illustrates an e-book reader 710 incorporating electronic
paper, which includes two housings, a housing 711 and a housing 712. The
housing 711 and the housing 712 include a display portion 714 and a
display portion 713, respectively. The housing 711 is connected to the
housing 712 by a hinge 715, so that the e-book reader can be opened and
closed using the hinge 715 as an axis.

[0148] The housing 711 is provided with operation keys 716, a power button
717, a speaker 718, and the like. At least one of the housings 711 and
712 is provided with the semiconductor device including the memory
element described in any of the above embodiments. Thus, an e-book reader
with sufficiently low power consumption, in which data can be stored for
a long time, can be obtained.

[0149]FIG. 8c is a television device 720, which includes a housing 721, a
display portion 722, a stand 723, and the like. The housing 721 can be
provided with the semiconductor device including the memory element
described in any of the above embodiments. Thus, a television device with
sufficiently low power consumption, in which data can be stored for a
long time, can be obtained.

[0150] As described above, the memory element in any of the above
embodiments is mounted on each of the electronic devices described in
this embodiment. Therefore, electronic devices with low power
consumption, in which data can be stored for a long time, can be
obtained. It is needless to say that a similar effect can be obtained
when an electronic device other than those illustrated in FIGS. 8A to 8C
includes the semiconductor device according to any of the above
embodiments.

[0151] This application is based on Japanese Patent Application serial No.
2011-117516 filed with Japan Patent Office on May 26, 2011, the entire
contents of which are hereby incorporated by reference.

Patent applications by Junichiro Sakata, Atsugi JP

Patent applications by SEMICONDUCTOR ENERGY LABORATORY CO., LTD.

Patent applications in class SEMICONDUCTOR IS AN OXIDE OF A METAL (E.G., CUO, ZNO) OR COPPER SULFIDE

Patent applications in all subclasses SEMICONDUCTOR IS AN OXIDE OF A METAL (E.G., CUO, ZNO) OR COPPER SULFIDE